The invention relates to a pixel sensor and more particularly to a digital pixel sensor (DPS) employing MOS transistors.
FIG. 1 is a circuit diagram illustrating a conventional analog pixel sensor. As shown in FIG. 1, the analog pixel sensor 1 includes a photodiode 10, a MOS transistor M1, a MOS transistor M2, and a MOS transistor M3.
When operating, the voltage of a reset signal RS is first pulled up to a high level, turning on transistor M1 and pulling the voltage at node PD up to VDD-Vth, where Vth is the threshold voltage of the transistor M1. Reset signal RS is then pulled down to a low level, turning off the transistor M1 and allowing the voltage of node PD to float. Photo diode 10 is illuminated by an incident light source, which produces a photocurrent of electron-hole pairs to flow in the photo diode 10. The electron field across the depletion region sweeps the electrons to node PD for storage, while concurrently, the holes are carried away by the current through the substrate. As a result of the discharge, the voltage at node PD falls gradually, with increased intensity of the incident light source, generating more electrons in photodiode 10, which speeds drop of voltage at node PD.
FIG. 2 is a relational diagram of the voltage Vout of the output signal OUT produced by the pixel sensor 1 with the voltage VRST of the reset signal RS, wherein the output signal OUT is read out through the transistors M2 and M3, controlled by a control signal SEL. As shown in FIG. 2, when the voltage VRST of the reset signal RS is at a low level, all voltages VOUT of the output signals OUT corresponding to different illumination intensities of the incident light source decays with time as shown by arrow 11. As illumination intensity increases, the Vout voltage drop speeds up. FIG. 3 further illustrates the voltage drop of the output signal OUT as a function of the illumination intensity of the incident light source. As shown in the figure, the voltage drop of VOUT increases with illumination intensity until it reaches a saturation state, the incident light source having a specific intensity Imax.
In the conventional pixel sensor 1, the output signal OUT representing the intensity of the incident light source is an analog signal, necessitating a conversion to a corresponding digital signal for successive processing. Obviously, if a digital signal can be output directly, both costs and processing time can be reduced.
FIG. 4 is a schematic diagram of a pixel sensor 4 producing a digital output signal, as proposed by Stanford Group. After the pixel sensor 4 is illuminated by an incident light source, the voltage at node D is transferred through a transmission gate (transistor) TX to the inverting node of a comparator 30 to act as an input signal IN, while a ramp signal Ramp is input to the non-inverting node of the comparator 30. When the voltage of the ramp signal Ramp meets that of the input signal, the output signal of the comparator 30 falls from the original high level to a low level, further driving a memory 31 to lock and store the number registered by a counter 31. Data stored in the memory 31 is then read out by a sensor amplifier (not shown). The digital pixel sensor, however, includes a large number of transistors, and therefore increases costs.